This invention relates to molded commutators for rotating electrical machinery. More particularly, it relates to a molded commutator having improved means for anchoring the commutator segments and to a method for fabricating such commutators.
Conventional commutators comprise a plurality of contact segments arranged in an overall cylindrical or annular shape, each segment separated by an insulated gap. The fabrication of commutators by means of a molding process has provided a relatively efficient and economical way of producing such commutators. For small, low speed commutators, where the centrifugal forces experienced by the contact segments in operation are not large enough to cause the commutator to break apart during use, conventional molded commutators are highly reliable. However, as the operative size or speed of rotation of the commutator increases, the centrifugal forces on the contact segments increase rapidly, the forces being roughly proportional to N.sup.2 R.sup.2 where N is the rotational speed of the commutator and R is the radius of the commutator. Thus, for some values of N.sup.2 R.sup.2, the centrifugal force on the commutator segments becomes greater than the strength of the plastic matrix used for molding, and such molded commutators cannot be used. It is known to provide claws, tangs, or similar protuberances on the interior surfaces of the conducting segments of such molded commutators in order to prevent the loosening and loss of the segments during rotation. A commutator typifying the prior art is disclosed in U.S. Pat. No. 3,987,539 issued to Gravener on Oct. 26, 1976. For such commutators, the contact segments are typically anchored by tangs embedded in the molded matrix. While such means of anchoring the segments are quite satisfactory for small commutators, they are not adequate for relatively larger ones, because the centrifugal force on the contact segments for larger commutators exceeds the mechanical strength of the molded matrix used to anchor the segments. To provide the strength required for large values of N.sup.2 R.sup.2, the large, high speed commutators currently available all employ a metal cap and cone design. Such designs are heavy and relatively expensive. Furthermore, the manufacture of metal cap and cone commutators is labor intensive and not easily automated, and therefore relatively expensive. Molded commutators, on the other hand, can be manufactured by processes which are less labor intensive and easily automated.
It is an object of the present invention to provide a new and improved commutator structure which provides an effective anchoring means suitable for large commutators.
It is a further object of the present invention to provide a commutator structure having sufficient strength for use in applications requiring large commutator diameters.
It is also an object of the present invention to provide a commutator structure which can be economically produced with existing technology.
It is still another object of the present invention to provide a method for manufacturing large commutators which is not labor intensive and can be easily automated.